This invention relates to roller- and vane-type pumps of the kind comprising a carrier mounted in a casing for rotation about a fixed axis, a cam ring mounted in the casing and extending about the carrier, the carrier having radially extending slots evenly spaced about its periphery in which rollers or vanes are disposed for radial movement in sealing engagement with the radially-inner cylindrical (cam) surface of the cam ring whereby chambers are formed between adjacent elements which increase and decrease in volume, as the carrier rotates about said axis, to pump fluid from the inlet port to the outlet port of the pump, the cam ring being mounted for guided movement in a direction substantially radially of the carrier thereby to vary the change in volume of the chambers in their pumping cycle and hence the delivery of the pump. At any instant at generally opposite diametral sides of the carrier two of the rollers or vanes form seals between respective adjacent circumferential ends of the inlet and outlet ports; when the number of rollers or vanes is even these two sealing rollers are diametrically opposite each other, but when the number is odd one of the sealing rollers is circumferentially displaced from the diametrically opposite position by half the angular pitch of the rollers.
The common practice in forming the cam rings of such pumps is to make the cam profile fully circular, which is the ideal profile only when the cam ring and carrier are concentric, i.e. when the pump is doing no useful work. Whenever the pump is doing useful work the two rollers or vanes which are instantaneously forming the seals between the inlet and outlet ports are moving along surfaces which are not concentric with the carrier and the two sealing rollers or vanes are therefore moving radially but to different extents and this leads to noisy operation which can be objectionable.